The above C.sub.21 dicarboxylic acids are known in the art. The C.sub.21 dicarboxylic acids are useful as surfactants, coatings and adhesives. Heretofore, the C.sub.21 dicarboxylic acids have been prepared from linoleic acid generally from a tall oil fatty acid source by reacting the linoleic acid portion in a fatty acid mixture such as tall oil fatty acids and acrylic acid in the presence of an iodine catalyst. The process being carried out by reacting the linoleic acid portion in a fatty acid mixture with acrylic acid in an amount up to 26% by weight of fatty acids and with an iodine catalyst in an amount from 0.01% to 0.5% by weight of the fatty acids at a temperature between 200.degree. C. and 270.degree. C. The fatty acid dicarboxylic acid mixture is then separated by distillation into an oleic-type fatty acid and C.sub.21 dicarboxylic acid.
Crystalline type clays have been used in dimerizing polyunsaturated monocarboxylic fatty acids. Typically, dimerization is accomplished by heating the polyunsaturated monocarboxylic fatty acids to a temperature of about 180.degree. C. to 260.degree. C.